Solid oxide cells (SOC's) generally include cells designed for different applications, such as solid oxide fuel cells (SOFC's), or solid oxide electrolysis cells (SOEC's). These types of cells are well known in the art. Typically, a solid oxide fuel cell comprises an electrolyte layer sandwiched by two electrode layers. During operation, usually at a temperature from about 500° C. to about 1100° C., one electrode is in contact with oxygen or air and the other electrode is in contact with a fuel gas.
Under typical operating conditions, a single solid oxide fuel cell produces less than 1 volt. To obtain high voltage and power from the SOFC's, it is therefore necessary to stack many cells together. Since the manufacturing methods for SOFC planar stacks comprise the manufacture of single cells, the obtained cells are subsequently stacked together with additional layers such as interconnects, current collectors, contact layers and seals to form a fuel cell stack suitable for the desired application.
One of the problems limiting the fuel cell mass production at an industrial level up to date is the high cost of the final cells. Therefore, for fuel cells and electrolysis cells operating in a medium temperature range (about 500° C. to 900° C.), it has been suggested to employ relatively cheap metal interconnect components separating the individual cells.
Suitable materials for said metal interconnect layers need to be oxidation resistant against anode and cathode gases under elevated operation temperatures, and must further exhibit a thermal expansion coefficient (TEC) that matches the TEC of the ceramic components of the cell. In view of these requirements, particularly chromia-forming alloys have been investigated as support materials during the manufacturing process, which will form the later interconnect layer. Said alloys comprise a high chromium content, i.e. about 15 to 22% by weight, which, under oxidative conditions, migrates towards the surface and forms a chromia barrier layer or chromia scale on the surface which protects the component against further oxidation. At the same time, said chromia scale exhibits a sufficiently high electrical conductivity in order not to impede the overall performance of the device.
However, during operation of the cell, chromium ions may diffuse from the chromium containing metal interconnect materials into the adjacent air electrode layers and disadvantageously affect the catalyst performance and thus limit the cell performance over time. This phenomenon is generally known as ‘chromium poisoning’. The chromium poisoning is due to the chromium in the metal interconnect being transported from the metal via gaseous chromium containing oxides and oxy-hydroxides and by surface diffusion on the bridging metal oxide components to the electrochemically active sites near to or on the air-side of the electrode, where they quickly deteriorate the electrochemical activity to a considerable degree.
In order to attain a low electrical resistance and to reduce chromium poisoning from the chromia scale forming alloys employed as the interconnect material, it has been suggested to apply a manganese chromium spinel on top of a layer of chromia. US-A1-2003/0059335 for example proposes a chromium oxide forming iron-based alloy, comprising 12 to 28 wt % chromium and small amounts of La, Mn, Ti, Si, and Al. The material is capable of forming at its surface a MnCr2O4 spinel phase at temperatures of 700° C. to 950° C. The so formed manganese chromium spinel was expected to have a slightly lower vaporization pressure for chromium containing species than chromia itself.
However, it was found that disadvantageously the Cr-diffusion in fact proceeds faster in the MnCr2O4 spinel than in the chromia layer. Thus, the formation of a duplex Cr2O3—(Mn,Cr)3O4 scale does not provide any significant increase in protection from chromium poisoning, as compared to a pure chromia scale.
Different coatings that prevent chromium containing vapours from developing on the employed iron-chrome-alloy used as the interconnect in cells have been discussed in both, patent and scientific journal literature, for example in Tietz, F. et al. (2004) DE 103 06 649 A1, Tietz, & Zahid (2004) DE 103 06 647 A1, Hilliard D. B. (2003) US2003194592-A1, Orlovskaya N et al. (2004). J. Am. Cer, Soc 87, pp. 1981-7, Chen, X. et al. (2004) Solid State Ionics 176, pp. 425-33. The common coating concept encompasses the formation of a spinel or perovskite layer with it's final microstructure on top of the metallic interconnect prior to the insertion in an SOFC stack, where said coating acts as a barrier layer to chromium migration from the metal interconnect into the air electrode compartment. In order for these coatings to be sufficiently tight, the adhesion between coating and substrate has to be perfect initially as well as in the longer terms, i.e. after thermal cycling, which puts severe limitations to both the processing and the match between thermal expansion coefficients of the layers and the steel.
In view of these problems, reactive coatings offer a better solution as they are transformed into a barrier layer grown at high temperature and typically exhibit a more gradual change in microstructure between the metallic substrate and the overlying oxide coating. Reactive coatings as such have been discussed in, for example, the reports on screening spinel and perovskite coatings: Fujiata, K. el al. (2004) J. Power Sources 131, pp. 261-9; and Larring & Norby (2000), J. of the Electrochem. Soc. 147, pp. 3251-6. The disclosed coatings are single-phase or single-substance coatings, and are applied in order to form an electrically conductive corrosion protective coating on the metallic surface. Said coatings contain as little chromium as possible in order to reduce the transport of chromium from the metal surface for as long as possible, and to reduce the transport of oxygen from the atmosphere into the metal.
Protective coatings functioning as chromium getters are disclosed, for example, in Jiang S. P et al. (2002) J. Eur Cer. Soc. 22, pp. 361-73; and Matsuzaki & Yasuda (2001) J of the Electrochem. Soc. 148, pp. A126-31. It is discussed how chromium-containing phases are precipitated on various air electrode materials and on different electrolyte materials. The interest particularly focuses on the chromium precipitated on air electrode materials including LSM and LSCF without cathodic polarization.
Disclosed is further that chromium readily bonds with elements, such as Mn, from the perovskite structure of the LSM cathode, and creates well-crystalline spinel phases which may be retrieved at the border surface between the electrolyte and air electrode material if the LSM air electrode is exposed to an electrochemical polarization. LSCF air electrodes are not as dependent upon the electrochemical potential of the electrode, but the chromium-containing phases nevertheless generally precipitate in the pore volume of the electrode.
In summary, the barrier layers suggested in the prior art so far function to suppress the formation of chromium containing vapours during operation of the cell from the beginning, but also require an additional and often costly processing of the metallic interconnects prior to their use in a SOFC stack, hence preventing mass production of the stacks. Furthermore, the applied surface coatings must be both, chemically and thermomechanically compatible with the metal interconnect materials in a wide range of temperatures, i.e. operating temperatures of the cell of about 500-900° C., which severely limits the use of otherwise suitable materials. Finally, while the initial chromium diffusion or evaporation may be reduced to certain extent, most layers cannot prevent chromium diffusion over the lifetime of the cell and thus cannot prevent chromium poisoning of the electrode layers over time.
Therefore, there still exists a need for a cost effective method for eliminating the evaporation of chromium from the interconnect surface in solid oxide cells, which makes it possible to accommodate chromium vapor species and which minimizes the electrical contact resistance between the interconnect and the air electrode layer.
In view of the above, the present invention advantageously provides reactive coatings, applied through a cheap method, such as spraying a particle suspension on the interconnect surface and/or the air side of the solid oxide cells prior to assembling of the cell stack, the coating minimizing the chromium evaporation from the metallic surface once a dense, preferably chromium-free reaction layer has been formed in a sufficient layer thickness. At the same time, an overlaying coating traps chromium-containing species diffusing from the metallic surface during the initial period before they can reach the active locations of the air electrode layer.
The multi-functional coatings of the present invention reduce the problem of chromium poisoning to a technologically insignificant level, at the same time avoiding the much more expensive application methods for applying coatings with a similar scope suggested in the prior art to date, and provide a cost effective multilayer barrier structure extending the life time of a SOC.